docs: def get_group_key_line(self, data, key):
          """Get the next group-style key's line number.
  
          :param data: string to parse
          :param key: the key category
          :returns: the found line number else -1
  
          """
          idx = -1
          for i, line in enumerate(data.splitlines()):
              if isin_start(self.groups[key], line):
                  idx = i
          return idx